In general, when CVD, etching, etc. are conducted on semiconductor wafers in the production of semiconductors such as semiconductor integrated circuits, etc., mass flow controllers are used to control the amount of a treating gas supplied at high accuracy.
FIG. 13 shows a conventional mass flow controller 2 disposed in the course of a path (for instance, gas pipe) 4 for flowing a fluid such as liquid, gas, etc. The mass flow controller 2 comprises a path 6 made of stainless steel, etc. with both ends connected to a gas pipe 4, a mass-flow-rate-sensing means 8 positioned on the upstream side of the path 6, a mass flow rate control valve mechanism 10 positioned on the downstream side of the path 6, and a mass-flow-controlling means 18 such as a micro-computer.
The mass-flow-rate-sensing means 8 comprises pluralities of bypass pipes 12, and a sensor pipe 14 having openings near both ends of the bypass pipes 12 to bypass the bypass pipes 12 for always flowing part of a gas at a predetermined ratio. A pair of series-connected resistors R1, R4 made of a material having resistivity changeable with temperature are wound around the sensor pipe 14. The resistor R1 is positioned on the upstream side of a gas flow, while the resistor R4 is on the downstream side. A sensor circuit 16 connected to the resistors R1, R4 outputs a mass flow rate signal Sg1.
The mass-flow-controlling means 18 calculates the mass flow rate of the gas according to the mass flow rate signal Sg1 output from the sensor circuit 16, to control the mass flow rate control valve mechanism 10 such that the mass flow rate is equal to a set mass flow rate indicated by a signal Sg0 input from outside.
The mass flow rate control valve mechanism 10 comprises a mass flow rate control valve 20 disposed on the downstream side of the path 6, and a circuit 28 for driving the mass flow rate control valve 20. The mass flow rate control valve 20 comprises a valve opening 24 disposed in the path 6, a metal diaphragm 22 for controlling the opening degree of the valve opening 24, an actuator 26 constituted by a laminated piezoelectric element fixed to an upper surface of the diaphragm 22, and a case 27 receiving the diaphragm 22 and the actuator 26. The valve-driving circuit 28 receives a driving signal from the mass-flow-controlling means 18 to output a valve-driving signal (voltage) S2 to the actuator 26, which deforms the diaphragm 22 to control the opening degree of the valve opening 24.
FIG. 14 shows the mass-flow-rate-sensing means 8. The sensor circuit 16 comprises two reference resistors R2, R3 parallel-connected to the resistors R1, R4, such that the series-connected resistors R1, R4 and the series-connected reference resistors R2, R3 constitute a bridge circuit. The reference resistors R2, R3 are kept at a constant temperature. The resistors R1, R4 function as a heater, too. This bridge circuit comprises a constant-current source 30 parallel-connected to the reference resistors R2, R3, and a differential circuit 32 whose inputs are connected to a connecting point of the resistors R1, R4 and a connecting point of the reference resistors R2, R3. The differential circuit 32 determines a mass flow rate from a potential difference between both connecting points, to output a mass flow rate signal Sg1.
When there is no gas flow passing through the sensor pipe 14, both resistors R1, R4 are at the same temperature, resulting in the bridge circuit in equilibrium, so that no potential difference is sensed by the differential circuit 32. When the gas flows at a mass flow rate Q through the sensor pipe 14, the gas is heated by the resistor R1 on the upstream side and flows to the resistor R4 on the downstream side, resulting in the movement of heat to generate temperature difference between the resistors R1, R4. As a result, the difference of resistivity is generated between both resistors R1, R4, so that potential difference proportional to the mass flow rate of the gas is sensed by the differential circuit 32. Accordingly, the mass flow rate signal Sg1 output from the differential circuit 32 is proportional to the mass flow rate of the gas. The degree of opening the mass flow rate control valve 20 is controlled by, for instance, a proportional-integral-derivative (PID) control method, such that the sensed mass flow rate of the gas is equal to a set mass flow rate (voltage signal Sg0).
In the mass flow controller 2 shown in FIG. 13, it is necessary that the flow rate of a gas actually passing through the mass flow rate control valve 20 is equal to the set mass flow rate represented by the signal Sg0 at high accuracy. However, variation with time, such as the attachment of foreign matter to an inner wall of the sensor pipe 14, etc., makes the flow rate of a gas actually passing through the mass flow rate control valve 20 slightly different from the time of installation even if the same valve-driving voltage S2 is applied.
FIG. 15 shows the mass flow controller 101 disclosed by JP8-335117A. This mass flow controller 101 comprises a pipe 111 disposed between an upstream-side pipe 103 connected to a fluid supply source 102 and a downstream-side pipe 105 connected to a vacuum pump 104; a variable valve 112, a pressure sensor 114, a temperature sensor 115, an ultrasonic nozzle 113 and a pressure sensor 116 mounted to the pipe 111 in this order from the upstream side; a control circuit 120 receiving the outputs of the pressure sensor 114, the temperature sensor 115 and the pressure sensor 116 via A/D converters; and a driver 121 receiving the output signal of the control circuit 120 to output a driving signal to the variable valve 112. With the pressure of a fluid on the upstream and downstream sides of the ultrasonic nozzle 113 set such that its Reynolds number at the ultrasonic nozzle 113 is 106 or more, the fluid can be supplied at a target mass flow rate without being affected by the pressure and temperature of the fluid on the downstream side. However, in even this mass flow controller 101, variation with time, such as the attachment of foreign matter to the ultrasonic nozzle 113, the inner surface wear of the nozzle, the drift of the pressure sensor 114 and the temperature sensor 115, etc. makes the actual flow rate of a gas slightly different from the time of installation, even if the same valve-driving voltage is applied.
U.S. Pat. No. 5,865,205 discloses a method for controlling a gas flow out of a reservoir of a known volume, comprising the steps of (a) providing a desired flow input signal and a calibration signal to a first circuit and producing a calibrated flow input signal; (b) providing the calibrated flow input signal to a flow control circuit, the flow control circuit producing a control signal to a flow control valve located in a gas flow path downstream of the reservoir to control the gas flow; (c) releasing a gas from the reservoir by opening a reservoir outlet isolation valve; (d) sensing the gas flow in the gas flow path at a location downstream of the flow control valve and providing a measured flow signal indicative thereof to the first flow control circuit; (e) calculating a desired mass of gas to be released from the reservoir by integrating the desired flow input signal over a period of time in which the reservoir outlet isolation valve is open and producing a first signal indicative thereof, (f) calculating an actual mass of gas released from the reservoir by comparing a first mass of gas residing in the reservoir at a first time prior to opening the outlet isolation valve, to a second mass of the gas residing in the reservoir at a second time after the output isolation valve is closed, and producing a second signal indicative thereof, and (g) comparing the first and second signals to produce an updated calibration signal. However, because this method uses the reservoir outlet isolation valve between the reservoir and the mass flow rate control valve, and a pressure-sensing means and an orifice between the mass flow rate control valve and the isolation valve, the overall structure of the apparatus is complicated, and the reservoir should have a large volume. Further, because the first and second signals each indicating a mass determined from the sensed pressure are compared, the mass signal is subjected to the same degree of variation with time (drift phenomenon and the change of a Cv value), resulting in errors easily occurring in the calibration results.
JP2006-38832A discloses a mass flow controller comprising a small tank for performing mass flow rate calibration by comparing pressure variation at calibration with a reference pressure variation. However, because a mass flow rate signal output from a mass-flow-rate-sensing means expected to undergo variation with time does not affect the calibration result, the calibration of the mass flow rate control does not necessarily have sufficient accuracy. Although it is assumed in this calibration method that a tank volume is constant, the tank volume actually changes for reasons such as the attachment of products to an inner wall of the tank, etc. Accordingly, the calibration result by this method suffers from errors by the variation of the tank volume.